Cornea cannot receive oxygen from the blood supply like other tissue. When the eye is open, the cornea primarily receives oxygen from the atmosphere, via the tears. When the eye is closed (e.g., during sleep), the cornea receives oxygen mainly from oxygen diffusion from the capillary plexus of the upper palpebral aperture vasculature. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation cause the undesirable growth of blood vessels in the cornea. Wearing of a soft contact lens inevitably reduces the oxygen supply to the cornea, because it can form an oxygen barrier that prevents oxygen from reaching the cornea. The oxygen transmissibility (Dk/t) of the contact lens worn by a patient, depending upon the oxygen permeability (Dk) of the lens material and the thickness (t) of the contact lens, is of vital importance for the oxygen supply to the cornea either from the atmosphere in the open eye state or from the capillary plexus of the upper palpebral aperture vasculature.
In recent years, soft silicone hydrogel contact lenses become more and more popular because of their high oxygen transmissibility and comfort. Silicone hydrogel (SiHy) contact lenses are made of a hydrated, crosslinked polymeric material that contains silicone and from about 10% to about 80% by weight of water within the lens polymer matrix at equilibrium. However, a SiHy contact lens may not have a very high oxygen permeability (e.g., greater than 180 Barrers). A very high oxygen permeability is likely required for alleviating the adverse effect of oxygen-impermeable electro-optic elements, which are incorporated in contact lenses (see, U.S. Pat. Nos. 6,851,805, 7,490,936 and 8,154,804), upon the permeation of oxygen through the contact lenses.
Silicone contact lenses, made essentially of a crosslinked silicone polymer (or a silicone rubber or elastomer), have been proposed previously (U.S. Pat. Nos. 3,916,033; 3,996,187, 3,996,189; 4,332,922; and 4,632,844, herein incorporated by references in their entireties), because of their very high oxygen permeability and good mechanical and optical properties. However, crosslinked silicone polymers generally are produced by crosslinking a silicone composition comprising (1) at least one polydiorganosiloxane having at least two alkenyl groups (e.g., vinyl group, allyl group, 1-propenyl group, and isopropenyl group) each bonded to the silicon atom of a siloxane unit, (2) at least one hydride-containing polydiorganosiloxane having at least two silane groups (one hydrogen atom bonded to the silicon atom of a siloxane unit), and (3) a hydrosilylation catalyst (e.g., a platinum group metal or its compounds), according to hydrosilylation reaction. This hydrosilylation crosslinking (curing) approach has several disadvantages. First, hydrosilylation crosslinking requires relatively long reaction time and thereby lower the production yield. Second, hydrosilylation reaction is performed at an elevated temperature with extended hours. If silicone contact lenses are produced by cast-molding in disposable plastic molds (e.g., polypropylene molds) in a mass-production manner currently used in contact lens industry for producing hydrogel or silicone hydrogel contact lenses, the harsh curing conditions (e.g., high temperature and extended hours) may cause significant issues in lens metrology. Third, the mechanical properties of silicone contact lenses are quit sensitive to the stoichiometry of silane and alkenyl groups in a silicone composition. It would be difficult to control this stoichiometry and to provide silicone contact lenses with adequate and consistent mechanical properties. Fourth, it would be a challenge to remove the hydrosilylation catalyst post molding so as to minimize or eliminate any toxicological risk.
Therefore, there is still a need for a method for producing silicone contact lenses having consistent mechanical properties in a cost-effective manner. There is also a need for silicone contact lenses with desired and consistent mechanical properties.